The invention relates to power semiconductor circuits, in one embodiment to power semiconductor circuits which are designed for switching inductive loads in self propelled applications.
Power semiconductor circuits are used, inter alia, as switches for supplying electric power to electric loads. For example, such a power semiconductor circuit, e.g., an inverter, may include one or more controllable power semiconductor switches, e.g., IGBTs or MOSFETs. In its ON-state, such a controllable power semiconductor switch may provide electric power to an electrical load.
During the transition phase from the ON- to the OFF-state induced voltages, caused by always existing parasitic stray inductances in the commutation loop, may reach levels dangerous for the controllable power semiconductor. Such voltages, if exceeding the rated break-down voltage of the controllable power semiconductor may lead to the destruction of the device.
One possibility to protect the controllable power semiconductor switch against such damage is active clamping (AC). To reduce an overvoltage affecting the controllable power semiconductor switch, during activated AC the load path of the controllable power semiconductor switch is switched back temporary to its ON-state during the transition of the controllable power semiconductor switch from the ON- to the OFF-state depending on the feedback signal of the AC circuitry.
However, in some applications, conventional AC technology is not applicable: For example, in a Hybrid Electrical Vehicle (HEV) with a permanent synchronous motor (PMSM). Assuming the HEV stays at the top of a hill and rolls then passively down the hill. As the PMSM is connected to the transmission it rotates and charges the DC-link capacitor through the freewheeling diodes of the inverter. If this back electromagnetic force (Back-EMF or BEMF) of the PMSM is higher than the designed activation voltage of the AC circuitry, the AC will be activated and turns on the controllable power semiconductor. Typically this leads to the destruction of the controllable power semiconductor by exceeding the short-circuit safe operating area (SC-SOA). Accordingly, implementation of a standard AC circuitry is not permitted. This scenario is quite unique to self propelling applications like HEV and electric vehicles (EV). Therefore, in standard industrial and consumer applications the need for a modified AC is typically not given.
Hence, there is a need for an improved method for protecting controllable power semiconductor switches particular for self propelled applications.